Fix typo and add more text to the error shown when 'wpdialogs' is not enqueued. Add 'wp_enqueue_editor' action fired when scripts and styles for the editor are being enqueued, fixes #16284

git-svn-id: https://develop.svn.wordpress.org/trunk@28024 602fd350-edb4-49c9-b593-d223f7449a82